(1) Field of the Invention
The present invention relates to an air ride suspension system for vehicles that can selectively raise and lower a vehicle axle supported by the suspension system relative to the vehicle, and can automatically adjust the pitch of the supported axle. In particular, the vehicle suspension system is automatically controlled to vary the pitch of a vehicle axle supported by the system between a forward and rearward caster, and is also selectively controlled by a vehicle driver to both raise and lower the vehicle axle relative to the vehicle.
(2) Description of the Related Art
Suspension systems of the type provided by the present invention are commonly employed in supporting a trailing axle or pusher axle of a vehicle. In conventional trailing or pusher axles, it is known that if the axle is installed with the proper pitch or caster, the drag exerted on the wheels supported by the axle will cause the wheels to steer automatically and follow the forward vehicle wheels in response to steering of the vehicle.
It is also known in the suspension system art to assemble suspension systems in the form of a parallelogram linkage assembly. Such parallelogram suspension systems are constructed as a parallelogram arrangement of torque rods connected between a hanger assembly and an axle seat. These systems commonly are comprised of a lower torque rod pivotally connected between the hanger and axle seat, and an upper torque rod pivotally connected between the hanger and axle seat. The parallelogram arrangement of the torque rods maintains the axle seat and the vehicle axle supported by the axle seat at a substantially constant pitch for all verticle displacements of the axle seat relative to the vehicle chassis.
Parallelogram linkage assemblies are also known which selectively elevate the axle seat relative to the vehicle chassis when the vehicle is supporting a reduced load. It is common for a selectively actuated air spring to be positioned between the lower torque rod and the hanger of the parallelogram system which selectively rotates the lower torque rod relative to the hanger to raise and lower the axle seat relative to the vehicle chassis.
Prior art vehicle suspension systems are known which combine a parallelogram arrangement that can selectively raise and lower a vehicle axle with the automatically controlled variable pitch axle. U.S. Pat. No. 4,770,430 to Lange discloses such a vehicle suspension system. A disadvantage of this suspension system and others of its kind is that one of the torque rods of the parallelogram linkage consists of a piston and cylinder assembly. The piston and cylinder assembly is used to vary the effective length of the torque rod, and thereby vary the pitch of the vehicle axle supported by the axle seat. However, the piston and cylinder assembly is not as rigid as the torque rod which it replaces. There are substantial forces exerted on an axle seat that has been lowered to an operative position to support some of the vehicle load. These forces will cause some fluctuation of the relative positions of the piston and cylinder of the variable length torque rod. The fluctuation in the relative positions of the piston and cylinder causes a corresponding fluctuation of the pitch of the axle supported by the axle seat. Because the difference in angular orientation of a vehicle axle adjusted between its forward and rearward pitch positions is very small, any fluctuation in the axle pitch could be effective to substantially negate the self steering effect obtained by setting the axle at a forward or rearward pitch.
It is an object of the present invention to overcome the aforesaid drawbacks of conventional parallelogram suspension systems by providing a parallelogram suspension system with an adjustable axle pitch that is more capable of resisting fluctuations of the axle pitch due to forces exerted on the vehicle axle than prior art systems. It is a further object of the present invention to provide a parallelogram suspension system that is selectively raised and lowered, and automatically adjusts the pitch of a vehicle axle supported by an axle seat of the system, where a piston and cylinder assembly is employed to vary the effective length of one of the torque beams of the system but the piston and cylinder assembly is not a part of the variable length torque beam.